1. Field of the Invention
The present invention relates to a vacuum heat insulator in which glass fibers are stacked at random in a receptacle which has a flat portion and is sealed under a vacuum condition.
2. Description of the Prior Art
A conventional vacuum heat insulator will now be described with reference to FIGS. 1 and 2. A vacuum heat insulator 1 is so constructed that glass fibers 5 of small diameters are stacked at random in a direction perpendicular to the heat transfer direction thereof within a plate-like receptacle 4 defined by a drawn thin steel plate or sheet 2 of low carbon and a planar stainless steel plate 3 while an interior of the receptacle 4 is sealed under a vacuum condition.
In such a vacuum heat insulator 1, since the receptacle 4 has a plate-like shape, when the interior of the receptacle 4 is made to be a vacuum condition, a great deforming force would be applied to the receptacle 4 at the planar portions of the two steel plates 2 and 3 thereof. On the other hand, since the glass fibers 5 are simply stacked at a low density in the receptacle 4, the glass fibers 5 are to be readily compressed by the deforming force applied to the receptacle 4, so that the receptacle 4 would be greatly deformed. Thus, there is a fear that it would be impossible to keep a space thickness required for the heat insulator. There is also a fear that upon deformation, a cracking would be generated in the receptacle 4 and it would be, therefore, impossible to keep the vacuum condition proper. For this reason, it is required that a degree of vacuum in the receptacle 4 is restricted and a deformation of the receptacle is to be suppressed. However, if the degree of vacuum is restricted, the insulating ability would be considerably degraded. In this case, since a density of glass fibers 5 becomes lower, a representative dimension of heat transfer of the space in the receptacle becomes large, so that a molecular free path is elongated and at the same time, the number of contacts between the glass fibers 5 becomes smaller, so that a thermal contact resistance becomes smaller. Due to such a phenomenon, the insulating ability would be unduly degraded.